The storage capacity and general performance of hard disk drives have improved steadily over the last decade in response to the increasing demands of the computer industry, logically evolving from mainframe to mini to work station to personal computer. These developments have been mainly fueled by corresponding advances in the components of magnetic recording technology and in electronic sophistication and miniaturization. In particular, magnetic read/write recording heads and disks are capable of supporting areal data storage densities typically ten times greater than they could ten years ago. This is being achieved by higher track densities, higher linear flux transition densities and lower recording head flying heights. The point is now being reached, however, where further advances are likely to be impeded by mechanical constraints. In particular, limitations will result from the spindle hub holding the stack of disks on account of the properties of the ball bearing assemblies used for rotational support. Random vibrations in both axial and radial axes ultimately lead to limitations in positioning accuracies and signal-to-noise ratios with consequences for storage capacities. The radial accuracy directly limits the number of concentric tracks on a disk on which data can be stored and reliably retrieved.
Design choices of ball bearing parameters such as type, pre-load, location, grease, etc., must also take aging into account. The deterioration with time and usage of ball bearings tends to degrade spindle performance. In addition, spindle frequency response variations, normally a function of rotational speed, number and quality of balls comprising the bearings, etc., must be reduced over the lifetime of the spindle since they are likely to contribute to positioning inaccuracies. Other consequences of the continued use of ball bearing-based spindles in very high performance disk drives include acoustic noise and relatively high power requirements.
The present invention is based on the technology of fluid films used to separate and support moving surfaces such that motion and load bearing may be achieved with great accuracy and consistency, with the minimum of influence from other factors. Typically, a fluid-film bearing is an anti-friction bearing in which rubbing surfaces are kept apart by a film of lubricant such as oil.
Devices which generate pressure by virtue of relative motion between surfaces are defined as being hydrodynamic or self-acting. Those which rely on an external source of pressure are called hydrostatic or pressurized systems. The latter have been used for many years in a wide variety of applications including, for example, machine tools. The self-acting bearing has the huge advantage of being self-contained with no requirement for external connection or supply, but puts much tighter constraints on design in order to generate adequate pressure. An example of this, used for many years in a disk drive itself, is the "flying" read/write head on the rotating, rigid disk. The typical head has a suitably contoured air bearing surface which permits it to take-off as the underlying disk begins to rotate and finally establish a stable flying position, e.g., 10 to 20 microinches above the surface, being supported by a relatively stiff air bearing acting against a mechanical spring. It should also be noted that self-acting, air bearing spindles have been, and are being, used in the disk drive industry in certain items of manufacturing and test equipment, where high accuracy is mandatory, such as servo track writers.
U.S. Pat. No. 4,200,344 (Binns et al) discloses a disk drive spindle employing a uni-directional, self-acting air bearing. The spindle operates with its axis vertical and uses an alternating current (AC) synchronous motor for rotation. The spindle load, however, is supported by a self-acting bearing using recirculating oil. In use, a disk pack containing up to 11 disks would be attached to the spindle with a combined rotating load of 50 pounds.
U.S. Pat. No. 4,487,514 (Mori) discloses a self-acting air bearing spindle without separate thrust air bearings and driven by an external motor.
IBM Technical Disclosure Bulletin, Vol. 29, No. 5 (1986), shows part of a spindle of a drive motor, to which recording disks could be attached. The spindle is supported radially by a self-acting air bearing and axially by a thrust bearing operating by means of a negative pressure air bearing.
An article entitled "Statistical methods reveal runout anomalies of disk spindles," by Bouchard et al, Computer Technology Review, No. 32, pages 32 and 33 (January 1987), is useful in that it presents results of experiments to measure and compare the non-repeatable run-out of disk spindles based on ball bearings, ferro-fluid bearings and air bearings.
There is thus a need for an improved motorized spindle assembly which can enable a new generation of disk storage products. The present invention fills that need by providing a self-acting, air bearing motorized spindle in a form-factor which allows it to directly replace standard ball bearing spindles in 8" and full height 51/4" rigid disk drives. The advantages of the present invention stem from the total absence of ball bearings, raceways, cages, pre-loads, etc., and their replacement by self-generating air bearings in both axial and radial axes. This results in almost unmeasurable vibration and random run-out, since the air bearings are effectively self-aligning, operate silently and are extremely reliable. There is effectively no wear. This smoothness of operation with consequent high accuracy is achieved by virtue of the averaging action of the air film which reduces the influence of geometric errors in the component surfaces, a situation contrasting directly with rolling contact found in ball bearings. The air bearing configuration chosen in a preferred embodiment of the present invention is a self-acting, inward pumping, herringbone groove-type which provides radial accuracies, when running at the appropriate design speed, at least several times better than the machining tolerance errors of the bearing surfaces supporting the high pressure film.
There are other advantages of the present invention over prior art devices which should be pointed out. The simplicity of the spindle in terms of the relatively small number of parts, consequent ease of assembly and self-aligning capability in operation imply that manufacturing costs in reasonably high volumes can be low, provided appropriate dedicated machine tools are utilized. Also, a primary feature of a preferred embodiment of the present invention is that the spindle is of the "in-hub" motor-type and externally indistinguishable from conventional bal bearing disk drive spindles. This is an important objective of the invention since it clearly enables ease of integration into the current designs and form-factors of the disk drive industry.